1. Field of the Invention
The present invention relates to an elastomer damper for abutting against a piston sliding in the bore of an actuator cylinder at its stroke end in order to absorb mechanical shock caused at the stroke end of the piston.
2. Description of the Related Art
An elastomer damper of this type is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 1-6505. In the elastomer damper in the '505 publication a damper member made of an elastomer is disposed in the actuator cylinder by attaching it to an end cap closing the end opening of the cylinder. More specifically, the damper member in the '505 publication has a cylindrical shape and has stepped diameter portions, i.e., a large diameter base and a smaller diameter tip portion. A recess for receiving the base of the damper member is formed on the inner end face of the end cap which faces inside of the cylinder. The damper member is fitted to the end cap by inserting the base of the damper member into the recess of the end cap. The recess has a diameter larger than that of the base so that, when the damper member is attached to the end cap, a clearance is formed between the outer periphery of the base and the wall of the recess. In order to fix the damper member in the recess, a fixing cap is fitted to the inner end face of the end cap. When the damper member is fitted to the end cap, the bottom face of the damper member firmly contacts the bottom of the recess and, in this condition, the tip of the damper member protrudes from the inner end face of the end cap in the direction toward the piston in the cylinder. A part of the tip portion of the damper member which protrudes from the inner end face of the end cap is tapered so that the diameter of this portion decreases in the direction toward the piston (i.e., in the direction towards the center of the length of the cylinder).
A groove for receiving a sealing member (such as an O-ring) preventing the leakage of the working fluid in the cylinder is defined by the fixing cap and the inner end face of the end cap. Further, the working fluid is supplied to and discharged from the cylinder through a fluid passage opening on the bottom surface of the recess of the end cap.
In the elastomer damper of the '505 publication, the damper member is compressed when the piston moving in the cylinder hits the damper member at its stroke end and, when it is compressed, the damper member expands in the radial direction in order to absorb the kinetic energy of the piston. However, since the damper member in the '505 publication consists of two portions, tip and base, having different diameters, this step wise change in the diameter causes a sudden change in the elasticity of the damper member when it is compressed by the piston. This sudden change in the elasticity causes a sudden change in the braking force exerting on the piston and hampers the smooth deceleration of the piston.
Further, the elastomer damper in the '505 publication requires the fixing cap in order to attach the damper member to the end cap and to form the receiving groove for O-ring. Since a separate element (i.e., the fixing cap) is required for fitting the elastomer damper and the O-ring, the number of the elements required for constructing the actuator increases.
Moreover, in the elastomer damper in the '505 publication, the top surface of the damper member which contacts the piston is formed as a flat plane and a sharp ridge is formed at the periphery of the top surface where the top surface meets the tapered side surface of the damper member. When the piston hits the top surface of the damper member, a stress concentration occurs at the ridge of the top surface. Therefore, after a long operating time, the ridge of the top surface tends to crack during the operation, and when a crack occurs at the ridge, it tends to extend toward the center of the top surface. This may cause a deterioration of the damping capability of the elastomer damper.
Further, the elastomer damper in the '505 publication is formed as a solid cylindrical body. Therefore, when the damper members compressed by the piston the damper member can expand only toward the outward radial direction. Thus, when a damper member having a large diameter is required, the amount of expansion in the radial direction becomes considerably large. In this case, a large clearance is required around the outer periphery of the damper member to accommodate the large amount of the radial expansion of the damper member. However, in a practical standpoint, it is not preferable to provide such a large clearance around the damper.